Estimation of the number of working hours critical for the development of mental and physical fatigue symptoms in Japanese male workers - application of benchmark dose method

BACKGROUND: To clarify the influence of working hours on subjective fatigue symptoms and obtain the critical dose (number of hours) to determine the number of permissible working hours, we calculated the benchmark dose (BMD) and the 95% lower confidence limit on BMD (BMDL) of working hours for subjective mental and fatigue symptoms using multivariate logistic regression. METHODS: Self-administered questionnaires were distributed to all 843 male daytime workers aged < or = 60 years in a single chemical factory, and 715 provided complete replies. The odds ratios of daily working hours were determined using positive findings of the Self-rating Depression Scale and 8 subscales of the Cumulative Fatigue Symptom Index as dependent variables, and other potential covariates as independent variables. Using significant parameters for the working hours and those for other covariates, the BMD and BMDL (BMD/BMDL) values were calculated for corresponding dependent variables. The benchmark response (BMR) was set at 5% or 10%. RESULTS: The BMDL with a BMR of 5% was shown to be 9.6-11.6 hr per day, which corresponds to 48-58 working hours per week and 36-78 overtime hours per month. CONCLUSIONS: These results suggest that special attention should be paid to the workers whose working hours exceed these BMD/BMDL values.